Optical filter device having creep-resistant optical fiber attachments

ABSTRACT

A method and device for tuning an optical device including an optical fiber having a core, a cladding and a Bragg grating imparted in the core to partially reflect an optical signal at a reflection wavelength characteristic of the spacing of the Bragg grating. The cladding has two variation regions located on opposite sides of the Bragg grating to allow attachment mechanisms to be disposed against the optical fiber. The attachment mechanisms are mounted to a frame so as to allow the spacing of the Bragg grating to be changed by an actuator which tunes the reflection wavelength. In particular, the variation region has a diameter different from the cladding diameter, and the attachment mechanism comprises a ferrule including a front portion having a profile substantially corresponding to diameter of the variation region and a butting mechanism butting the ferrule against the optical fiber.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a Continuation-In-Part application of co-pending U.S. patentapplication Ser. No. 09/073,701 entitled “Creep-Resistant Optical FiberAttachment”, filed May 6, 1998, which is related to and filed on evendate with U.S. patent application Ser. No. 09/073,700 entitled “OpticalFiber Bulge”, which is now abandoned, and U.S. patent application Ser.No. 09/073,699, entitled “Optical Fiber Outer Dimension Variation”,which is also abandoned. U.S. patent application Ser. No. 09/073,701 hasbeen published under the Patent Cooperation Treaty (PCT) on Nov. 11,1999 as International Publication No. WO 99/57589. This patentapplication is also related to patent application Ser. No. 09/873,978,assigned to the assignee of the present invention and filed on even dateherewith.

TECHNICAL FIELD

The present invention generally relates to fiber gratings and, moreparticularly, to a tunable Bragg grating and laser.

BACKGROUND ART

It is known in the art of fiber optics that Bragg gratings embedded inthe fiber may be used in compression to act as a tunable filter ortunable fiber laser, as is described in U.S. Pat. No. 5,469,520,entitled “Compression Tuned Fiber Grating” to Morey, et al and U.S. Pat.No. 5,691,999, entitled “Compression Tuned Fiber Laser” to Ball et al,respectively, which are hereby incorporated herein by reference.

To avoid fiber buckling under compression, the technique described inthe aforementioned U.S. Pat. Nos. 5,469,520 and 5,691,999 uses slidingferrules around the fiber and grating and places the ferrules in amechanical structure to guide, align and confine the ferrules and thefiber. However, it would be desirable to obtain a configuration thatallows a fiber grating to be compressed without buckling and withoutsliding ferrules and without requiring such a mechanical structure.

Also, it is known to attach an optical fiber grating to within a glasstube to avoid buckling under compression for providing awavelength-stable temperature compensated fiber Bragg grating, as isdescribed in U.S. Pat. No. 5,042,898, entitled “Incorporated BraggFilter Temperature Compensated Optical Waveguide Device”, to Morey etal. However, such a technique exhibits creep between the fiber and thetube over time, or at high temperatures, or over large compressionranges.

SUMMARY OF THE INVENTION

The first aspect of the present invention is a tunable optical device,which comprises an optical waveguide having a longitudinal axis, a firstmounting location and a second mounting location separated by a distancealong the longitudinal axis, which transmits an optical signal, whereinthe waveguide comprises a core and a cladding disposed outside the core,and wherein the cladding has an outside diameter and includes a firstand a second variation region each having a modified outside diameterdifferent from the outside diameter, wherein the first and secondvariation regions are respectively located at the first mountinglocation and the second mounting location, a Bragg grating imparted inthe core of the waveguide between the first mounting location and thesecond mounting location, wherein the Bragg grating comprises aplurality of perturbations defined by a spacing along the longitudinalaxis to partially reflect the transmitted optical signal at a reflectionwavelength characteristic of the spacing of the Bragg grating, a firstattachment mechanism disposed against at least one portion of the firstvariation region which prevents relative movement between the firstvariation region and the first attachment mechanism, a second attachmentmechanism disposed against at least one portion of the second variationregion which prevents relative movement between the second variationregion and the second attached mechanism, a mounting device having afirst end for fixedly mounting the first attachment mechanism and asecond end which movably mounts to the second attachment mechanism anddefines a separation length between the first and second attachmentmechanisms along the longitudinal axis of the waveguide, and anadjustment mechanism, operatively connected to the second attachmentmechanism, which adjusts the separation length, thereby causing a changein the distance between the first and second variation regions and thespacing of the Bragg grating to tune the reflection wavelength.

According to the present invention, the attachment mechanism comprises afirst ferrule including a front portion having a profile substantiallycorresponding to the modified outside diameter of the first variationregion of the cladding and a first butting mechanism butting the firstferrule against the waveguide to press the front portion of the firstferrule onto at least one portion of the first variation region at thefirst mounting location which limits relative movement between the firstferrule and the first variation region of the cladding, and the secondattachment mechanism comprises a second ferrule including a frontportion having a profile substantially corresponding to the modifiedoutside diameter of the second variation region of the cladding and asecond butting mechanism butting the second ferrule against thewaveguide to press the front portion of the second ferrule onto at leastone portion of the second variation region at the second mountinglocation which limits relative movement between the second ferrule andthe second variation region of the cladding.

According to the present invention, the first butting mechanism providesa pressing force against the front portion of the first ferrule along afirst direction substantially parallel to the longitudinal axis, and thesecond butting mechanism provides a pressing force against the frontportion of the second ferrule along a second direction substantiallyopposite to the first direction.

According to the present invention, the waveguide further comprises abuffer layer over the cladding to protect the waveguide against thefirst and second attachment mechanisms and which enhances attachment ofthe first and second attachment mechanisms to the waveguide.

According to the present invention, the first and second ferrulescomprise a plurality of pieces substantially surrounding the respectivevariation regions, which attach to the cladding.

According to the present invention, wherein a further waveguide segmentincluding a cladding having a second outside diameter substantiallyequal to the modified outside diameter is spliced with the waveguide inorder to provide each of the first and second variation regions.

According to the present invention, the modified outside diameter isprovided by heating and stretching the waveguide to change the outsidediameter of the cladding.

According to the present invention, the optical waveguide is an opticalfiber.

According to the present invention, the adjustment mechanism can be apiezoelectric transducer, a stepping motor, a pneumatic force actuator,a solenoid or the like.

Furthermore, a section of the core between the variation regions,including the Bragg grating, is doped with a rare-earth dopant forforming a laser with the Bragg grating.

The second aspect of the present invention is a method of wavelengthtuning an optical, wherein the optical device comprises an opticalwaveguide having a longitudinal axis to transmit an optical signal,wherein the waveguide has a first mounting location and a secondmounting location separated by a distance along the longitudinal axis,and wherein the waveguide comprises a core and a cladding disposedoutside the core; wherein the cladding has an outside diameter andincludes a first and a second variation region each having a modifiedoutside diameter different from the outside diameter, and wherein thefirst and second variation regions are respectively located at the firstmounting location and the second mounting location; and a Bragg gratingimparted in the core of the waveguide between the first mountinglocation and the second mounting location, wherein the Bragg gratingcomprises a plurality of perturbations defined by a spacing along thelongitudinal axis to partially reflect the transmitted optical signal ata reflection wavelength characteristic of the spacing of the grating,said method comprising the steps of:

providing a first attachment mechanism disposed against at least oneportion of the first variation region which prevents relative movementbetween the first variation region and the first attachment mechanism;

providing a second attachment mechanism disposed against at least oneportion of the second variation region which prevents relative movementbetween the second variation region and the second attached mechanism;

providing a mounting device having a first end which fixedly mounts tothe first attachment mechanism and a second end which movably mounts tothe second attachment mechanism in order to define a separation lengthbetween the first and second attachment mechanisms along thelongitudinal axis of the waveguide; and

providing an adjustment mechanism, operatively connected to the secondmechanism, to adjust the separation length, thereby causing a change inthe distance between the first and second variation regions and thespacing of the grating which tunes the reflection wavelength.

According to the present invention, the first attachment mechanismcomprises a first ferrule including a front portion having a profilesubstantially corresponding to the modified outside diameter of thefirst variation region of the cladding and a first butting mechanismbutting the first ferrule against the waveguide to press the frontportion of the first ferrule onto at least one portion of the firstvariation region at the first mounting location in order to limitrelative movement between the first ferrule and the first variationregion of the cladding; and the second attachment mechanism comprises asecond ferrule including a front portion having a profile substantiallycorresponding to the modified outside diameter of the second variationregion of the cladding and a second butting mechanism butting the secondferrule against the waveguide to press the front portion of the secondferrule onto at least one portion of the second variation region at thesecond mounting location in order to limit relative movement between thesecond ferrule and the second variation region of the cladding.

According to the present invention, the method further comprises thestep of providing a coating between the cladding and the first andsecond ferrules which helps the ferrules to conform with the outsidediameter of the respective variation regions in order to reduce pointcontact stresses on the waveguide.

According to the present invention, the method further comprises thestep of providing a buffer layer over the cladding which protects thewaveguide against the first and second attachment mechanisms andenhances attachment of the first and second attachment mechanisms to thewaveguide.

According to the present invention, the method further comprises thestep of bonding the buffer layer to the first and second attachmentmechanisms.

According to the present invention, the method further comprises thestep of splicing a further waveguide segment including a cladding havinga second outside diameter substantially equal to the modified outsidediameter with the waveguide to form each of the first and secondvariation regions.

According to the present invention, the method further comprises thestep of heating and stretching the waveguide to form the modifiedoutside diameter of the first and second variation regions.

The present invention provides a significant improvement over the priorart by combining an optical fiber, having an expanded and/or recessedouter dimension variation region, with a structure, such as a ferrule orhousing, having a size and shape such that the structure mechanicallylocks against at least a portion of the variation, thereby allowing thestructure to attach to the fiber with minimal relative movement (orcreep) in at least one predetermined direction between the fiber and thestructure. The variation region and the structure may have variousdifferent shapes and sizes. However, while the geometry of the variationregion is created from the optical fiber, low optical loss of the lightbeing transmitted through the core of the fiber is maintained. There mayalso be a buffer layer between the cladding and the ferrule to protectthe fiber and/or to help secure the structure to the fiber to minimizecreep. Adhesives, such as solders, brazes, epoxies, etc., may also beused between the structure and the variation region.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is side view cross-section of a tunable Bragg grating, accordingto the present invention.

FIG. 2a is a diagrammatic representation illustrating a piezoelectrictransducer being used for tuning the Bragg grating, according to thepresent invention.

FIG. 2b is a diagrammatic representation illustrating a stepper motorbeing used for tuning the Bragg grating, according to the presentinvention.

FIG. 2c is a diagrammatic representation of an alternative embodiment ofthe tunable Bragg grating, according to the present invention.

FIG. 2d is a diagrammatic representation of another alternativeembodiment of the tunable Bragg grating, according to the presentinvention.

FIG. 3 is a side view cross-section of an optical fiber with anincreased diameter region and an attachment mechanism engaged therewith,in accordance with the present invention.

FIG. 4 is a side view cross-section of an optical fiber with anincreased diameter region and another attachment mechanism engagedtherewith, in accordance with the present invention.

FIG. 5 is a side view cross-section of an optical fiber with anincreased diameter region and yet another attachment mechanism engagedtherewith, in accordance with the present invention.

FIG. 6 is a side view cross-section of an optical fiber with anincreased diameter region and an attachment mechanism having a ferrulestraddling the region, in accordance with the present invention.

FIG. 7 is a side view cross-section of an optical fiber with anincreased diameter region having a straight geometry and an attachmentmechanism engaged therewith, in accordance with the present invention.

FIG. 8 is a side view cross-section of an optical fiber with anincreased diameter region having a notch and a ferrule adjacent thereto,in accordance with the present invention.

FIG. 9 is a side view cross-section of an optical fiber with a decreaseddiameter region and an attachment mechanism engaged therewith, inaccordance with the present invention.

FIG. 10 is a side view cross-section of an optical fiber with adecreased diameter region and another attachment mechanism engagedtherewith, in accordance with the present invention.

FIG. 11 is a side view cross-section of an optical fiber showing atechnique for creating an increased diameter region in an optical fiber,in accordance with the present invention.

FIG. 12 is a side view cross-section of an alternative technique forcreating an increased diameter region in an optical fiber, in accordancewith the present invention.

FIG. 13 is a side view cross-section of yet another technique forcreating an increased diameter region in an optical fiber, in accordancewith the present invention.

FIG. 14 is a side view cross-section of an alternative technique forcreating a decreased diameter region in an optical fiber, in accordancewith the present invention.

FIG. 15 is a perspective view of a device that may be used to create anincreased diameter region in an optical fiber, in accordance with thepresent invention.

FIG. 16 is a blown-up perspective view of a heating filament used toheat an optical fiber, in accordance with the present invention.

FIG. 17a a is a diagrammatic representation of a tunable fiber laserhaving two Bragg gratings to form a cavity, according to the presentinvention.

FIG. 17b is a diagrammatic representation of a distributed feedbackfiber laser, having a single Bragg grating, according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a tunable Bragg grating 110 comprises a single-modeoptical waveguide or optical fiber 10 having a core 14 and a cladding12. The cladding 12 has two variation regions 16, 16′ and a Bragggrating 180 imparted in the core 14 between the variation regions 16,16′. A frame 120 has a first end 122 and a second end 124 for mounting,respectively, a first attachment mechanism 200 disposed against theoptical fiber 10 at a first mounting location 142, and a secondattachment mechanism 200′ disposed against the optical fiber 10 at asecond mounting location 144. The first attachment mechanism 200 isfixedly mounted on a mounting member 202 at the first end 122 of theframe 120, and the second attachment mechanism 200′ is fixedly mountedon the mounting member 202 at the other end 124 of the frame 120. Anactuator mechanism 130 is mounted between the second end 124 of theframe 120 and the second attachment mechanism 200′. The distance Sbetween the variation regions 16 and 16′ can be changed by adjusting theseparation between the first attachment mechanism 200 and the secondattachment mechanism 200′. Bragg gratings are well known. The Bragggrating 180 has a plurality of “fringes” 182 formed from perturbationsin the refractive index of the core 14, such as that described in U.S.Pat. Nos. 4,725,110 and 4,807,950, entitled “Method for ImpressingGratings Within Fiber Optics”, to Glen et al., which is herebyincorporated by reference to the extent necessary to understand thepresent invention. The perturbations in the fiber core 14 are defined byspacing δ for partially reflecting an optical signal 190 transmitted inthe optical fiber 10. The reflected signal is denoted by referencenumeral 192. The remaining wavelengths of the optical signal propagatingthrough the Bragg grating 180 is denoted by reference numeral 194. TheBragg grating 180 is used to selectively reflect a particular frequencyor wavelength of light that is propagated along the core 14. Theparticular wavelength of light reflected by the Bragg grating 180 isuniquely determined by the grating spacing δ. When used intelecommunications, it is preferred that the reflection wavelength istunable. Accordingly, it is preferred that the spacing δ of the Bragggrating 180 can be adjusted by shortening or lengthening the distance S.As shown in FIG. 1, the actuator mechanism 130 is used to exert a forceF along the longitudinal axis 140 of the fiber 10 for pushing the secondattachment mechanism 200′ toward the first attachment mechanism 200 toshorten the distance S for compression-tuning the Bragg grating 180. Itis also possible to exert a force F for pulling the second attachmentmechanism 200′ away from the first attachment mechanism 200 to lengthenthe distance S. Furthermore, the mounting member 202 can be made as aseparate piece from the attachment mechanisms 200, 200′, but theattachment mechanisms 200, 200′ and the mounting member 202 can be anintegral piece of material.

FIG. 2a shows a method of adjusting the spacing δ of the Bragg grating180 (FIG.1) by using a piezoelectric transducer or actuator 132, whichis connected to a voltage source V, for exert a force on the secondattachment mechanism 200′. Alternatively, a stepper motor 136 connectedto a controller 138 can be used to adjust the position of the secondattachment mechanism 200′ in relation to the second end 124 of the frame120, as shown in FIG. 2b. It is also possible to use a solenoid, apneumatic force actuator, or any device, which is capable of directly orindirectly applying an axial force on the second attachment mechanism200′.

FIG. 2c shows an alternative embodiment of the present invention. Asshown in FIG. 2c, the first attachment mechanism 200 is fixedly mountedto the frame 120 with fastening means, such as screws 127. The frame 120also has blocking means 125 based on which the actuator 132 pushes andpulls the second attachment mechanism 200′, if it is desired to lengthenor shorten the distance S between the two variation regions. 16, 16′.

FIG. 2d shows yet another embodiment of the present invention. As shownin FIG. 2d, the actuator 130 is placed between the first attachmentmechanism 200 and the second attachment mechanism 200′. It is preferableto leave a gap 128 between the first attachment mechanism 200 and thefirst end 122 of the frame 120, and a gap 128′ between the secondattachment 200′ and the second end 124 of the frame. As such, it ispossible for the actuator 130 to push or pull the first attachmentmechanism 200 and the second attachment mechanism 200′ simultaneously tolengthen or shorten the distance S between the two variation regions 16,16′.

Compression-tuned Bragg grating has been described in U.S. patentapplication Ser. No. 09/456,112 entitled “Compression-tuned BraggGrating and Laser” by Mark R. Fernald et al. The main object of thepresent invention is to make tunable Bragg gratings and fiber lasersmore reliable by providing a method and system for firmly attaching theoptical fiber 10 to a pair of attachment mechanisms 200, 200′ disposedin a frame so that the changes in the spacing δ of the Bragg grating canbe achieved by adjusting the separation distance between the attachmentmechanisms. In particular, according to the present invention, theattachment mechanisms 200, 200′ are attached to the optical fiber 10 attwo mounting locations 142, 144. At each of the mounting locations, avariation region 16, 16′ of the cladding 14 is provided so that theattachment mechanism can be firmly disposed against the cladding 14. Ingeneral, the outside diameter of cladding at the variation region 16,16′ is different from the outside diameter of cladding in other parts ofthe optical fiber 10. The variation region can be an expanded region ora recessed region and have various shapes. The object is to preventrelative movement between the attachment mechanism and the optical fiberat the mounting location. FIGS. 3 to 10 illustrate various embodimentsof the attachment mechanism, according to the present invention.

Referring to FIG. 3, the cladding 12 of the optical fiber 10 has anouter diameter d1 of about 125 microns and the core 14 has a diameter d2of approximately 7-10 microns (e.g., 9 microns). The fiber 10 isdesigned to propagate light along the core 14 of the fiber 10. Thecladding 12 and the core 14 are made of fused silica glass or dopedsilica glasses. Other materials for the optical fiber or waveguide maybe used if desired. The fiber 10 has a region 16 with an expanded (orincreased) outer diameter (or dimension). The expanded region 16 has alength L of about 500 microns, and an outer diameter d3 of about 200microns. Other dimensions of the cladding 12, core 14, and expandedregion 16 may be used if desired, provided the diameter d3 of theexpanded region 16 is greater than the diameter d1. Also, the fiber 10may have an outer coating or buffer layer 18 used to protect the fiberand/or enhance attachment to the fiber (discussed more hereinafter).

The region 16 may be made by any technique for making a variation in anouter dimension of a waveguide. Some techniques for making the region 16are described in conjunction with FIGS. 11-14 below. A device forcreating an expanded region 16 is described in conjunction with FIGS. 15and 16. The region 16 allows the fiber 10 to be attached to a structurein many different ways, as described hereinafter with FIGS. 3-10.

In particular, referring to FIG. 3, a ferrule 30 (or sleeve) may bebutted (or mated) against at least a portion of the expanded region 16to provide a mechanical stop (or lock), which substantially prevents thefiber 10 from moving to the left relative to the ferrule 30, asindicated by a line 20 (i.e., the direction of an applied load on thefiber 10). The ferrule 30 may have a generally cylindrical and/orconical shape, or other shapes as discussed more hereinafter. Theferrule 30 may also overlap all or a portion of the expanded region 16.It is not required for the ferrule 30 to overlap the expanded region 16;however, overlap reduces point contact stresses on the fiber/expandedregion, to reduce the possibility of cracking the glass of the fiber 10and/or the expanded region 16, particularly when the expanded region 16has a curved geometry.

Referring again to FIG. 3, the ferrule 30 has a front region 32 with ageometry (shape, contour, or profile) that substantially corresponds tothe geometry of the expanded region. The shape of the region 32 need notexactly match that of the expanded region 16, and may be a straighttaper or bevel instead of a curved surface. Also, the ferrule 30 mayhave a beveled section 34 to provide some stress relief on the fiberwhen the fiber 10 flexes or is pulled off-axis from the ferrule 30.Instead of the taper 34, the ferrule 30 may be terminated with a sharpperpendicular edge, if so desired.

The ferrule 30 may be butted directly against the expanded region 16 ormay be bonded to the fiber 10 and/or the expanded region 16 with anadhesive material discussed hereinafter. The ferrule 30 may bepre-formed such that the shape of the front region 32 substantiallyconforms to the geometry of the expanded region 16. However, if theshape of the ferrule 30 does not match that of the expanded region 16, amaterial, e.g., an adhesive, a coating and/or a filler (discussed morehereinafter), may be used to fill any gaps therebetween to reduce pointcontact stresses on the fiber/expanded region and/or to provide bondingtherebetween.

Alternatively, the ferrule 30 may be heated and/or pressure (or force)applied to the ferrule 30, e.g., by atmospheric (such as pressure and/orvacuum), mechanical (such as crimping), and/or magnetic techniques (suchas electromagnetic forming), or any other technique, as indicated byarrows 42, to cause the ferrule 30 to match at least a portion of thegeometry of the expanded region 16. For a glass ferrule, the ferrule 30may be heated to a temperature at or below the softening temperature ofthe glass. For a metal ferrule, the ferrule may be heated to atemperature where the metal can be shaped. Alternatively, the ferrule 30may be heated, held in place, and the fiber 10 pulled longitudinallytoward and into contact with the ferrule 30 to force the ferrule 30 toconform to the shape of the variation region 16. This is particularlyuseful when the ferrule 30 is made of a metal, which has a much lowersoftening temperature than the glass, but may be used for any ferrulematerial.

Alternatively, the ferrule 30 may have a section 38, which extends tothe right of and overhangs the right side of the expanded region 16. Inthat case, the region 40 between the inside of the ferrule 30 and theright side of the expanded region 16 may be partially or completelyfilled with an adhesive, e.g., solder, braze, epoxy, etc., similar tothose discussed hereinafter. The adhesive may also fill any gaps on theleft side of the region 16 along the region 32. In that case, to helpminimize creep, the adhesive should be localized to the fiber variationregion 16, and, thus, avoiding putting the adhesive in a region 33 willhelp avoid creep in the region 33. Instead of filling the region 40 withan adhesive, the section 38 may be heated and/or pressure (or force)applied to the section 38 (using any of the techniques discussedhereinbefore with the arrows 42), as indicated by arrows 44, to forcethe ferrule section 38 to conform to at least a portion of the rightside of the expanded region 16. Also, such heating and/or applying ofpressure (or force) may be performed on the regions 32,38 together,e.g., with a single crimping tool, coining tool, or the like.

The ferrule 30 may be made of a ceramic/glass (e.g., sapphire, ruby,fused quartz, fused silica, etc.), a metal (e.g., Invar (64%Fe, 36%Nialloy), or Kovar (54%Fe, 29Ni, 17%Co)), or other low thermal expansionmaterials. The thermal expansion coefficient of the ferrule 30 should beclose to that of the optical fiber 10 so that the geometry of theferrule 30 and the expanded region 16 and/or the fiber 10 willsubstantially track each other over temperature to minimize creep andpoint contact stresses. If the optical fiber comprises silica glass (andthus has a low thermal expansion), a low thermal expansion material isdesirable for the ferrule 30. Other optical fiber or waveguide materialsmay be used if desired, with the material for ferrule 30 being selectedto have a substantially similar thermal expansion coefficient.

For any of the embodiments described herein, the ferrule 30 may bebonded to the fiber 10 and/or expanded region 16 using epoxy, metalsolders, metal brazes, glass solders, ceramic adhesives, or otheradhesive materials depending on the ferrule material, the fibermaterial, and whether or not there is the outer buffer layer (orcoating) 18 on the cladding 12 of the fiber 10. Alternatively, asdiscussed hereinbefore, the ferrule may be butted-up against the region16 without any adhesives.

Also, the buffer layer 18 (if used) may be made of various materials,e.g., metal, polymer, teflon, and/or cargon, or other materials, and maycomprise a plurality of layers. The buffer layer 18 may be used toprotect the fiber, and/or enhance attachment of the ferrule 30 to thefiber (e.g., reduce creep). The buffer layer 18 may comprise a metallayer (or metalized coating) made of a material that is rigid enough toprotect the outer surface of the fiber to help prevent fiber breakage ator near the region 16. The metal layer may also be a material that ismaleable (i.e., a material that deforms plastically under a compressiveload) that can sustain local compressive loads and exhibits high plasticstrain without material failure (e.g., tearing, forming voids, etc.),which helps the ferrule 30 conform to the geometry of the region 16.Some such maleable metals include gold, platinum, nickel, etc. Also, themetal layer may be used to promote glass surface wetting for solders.

For example, the buffer layer 18 may be made of nickel-gold (NiAu),having a thickness of about 1-3 microns Ni on the fiber and about 70-150nanometers Au on the nickel, or thicker Au, e.g., about 1-10 microns,may be used. Such a buffer layer 18 may be used with metal solder tosolder the ferrule 30 to the layer 18, or may be used without any solder(where the ferrule is butted-up against the region 16). Other metals andthickness may be used for the metal layer.

Alternatively, the buffer layer 18 may comprise a layer of polymer(e.g., high temperature polyimide) having a thickness of about1-10microns over the metal layer or directly on the cladding 12 withoutthe metal layer. Other types of polymers and thickness may be used. Whena polymer is used, it may be necessary to heat the combinedfiber/variation/ferrule to an elevated temperature (e.g., at or abovethe operating temperature for the application) for a settling time, toallow the polymer to reach a steady state condition, e.g., thickness,shape, displacement, etc. and thus exhibit minimal creep. Otherthicknesses, a number of layers, materials and compositions of thelayers of the buffer layer 18 may be used.

Also the ferrule 30 may have an inner diameter coating 41 of one or moreof the aforementioned maleable material(s), to help the ferrule 30 toconform to the geometry of the expanded region 16, thereby reducingpoint contact stresses on the fiber, and/or to enhance bonding to thebuffer layer 18 or to the fiber 10. Such an inner coating on the ferrule30 may be used whether or not the buffer layer 18 on the fiber 10 isused, and whether or not the ferrule 30 is soldered to the fiber 10 orthe expanded region 16. Also, the inner diameter of the ferrule 10 maybe polished to reduce stress concentrations.

The ferrule 30 may be a one-piece ferrule, or a semi-circular two ormore piece ferrule. Using a multi-piece ferrule provides the advantageof not having to slide the ferrule 30 along the fiber 10 to the region16, thereby reducing the possibility of scratching or causing otherdamage to the outer surface of the fiber 10 (with or without the buffercoating 18) and allowing the buffer layer 18 away from the region 16 tobe thicker and/or non-uniform.

Referring to FIG. 4, in an alternative design of the ferrule 30, thesection 38 extends to the right of and overhangs the right side of theexpanded region 16 and some of the fiber 10. If the ferrule 30 is amulti-piece ferrule, to hold such a ferrule together (around the fiber10 and/or the region 16), the ferrule 30 may be self-locking, may behinged (like a clamshell), and/or a collar 46 may be used. The collar 46may have a substantially straight inner diameter d5, of, e.g., 0.022inches, which substantially matches or is slightly less than the outerdiameter of the ferrule 30 to provide a contacting or frictional fitbetween the collar 46 and the ferrule 30. Other ferrule and collardiameters or dimensions may be used if desired. Also the ferrule 30 mayhave an enlarged region 47 to provide a stop for the collar 46 or forother purposes. Further, the ferrule 30 may have a recessed region 48 toallow for the collar 46 and ferrule 30 to mate flush at a face 45. Also,a slight bevel 49 may be provided on the collar 46 or the ferrule 30 toallow a tool, e.g., a razor blade, to be inserted to separate the collar46 from the ferrule 30. The collar 46 may be made of the same materialas the ferrule 30, or a material with substantially the same thermalexpansion coefficient. Alternatively, the collar 46 may be made of heatshrinking materials such as metals, polymers, or shape memory alloys. Tofacilitate assembly of the multi-piece ferrule 30 onto the fiber 10, themulti-piece ferrule 30 may be placed into the collar 46 and then slidalong the fiber 10 to the region 16. Then, heating and/or applyingpressure (or force) to the collar 46 (such as discussed hereinbeforewith FIG. 3 with the ferrule 30) will cause at least a portion of theferrule 30 to lock onto at least a portion of the region 16.

The length L2 of the ferrule 30 is about 0.075 inches. Other longer orshorter lengths may be used for the ferrule 30.

Referring to FIG. 5, instead of the ferrule 30 having a long cylindricalshape, it may be shorter and/or wider and may resemble a washer, bead orbearing jewel. For example, the ferrule 30 in FIG. 5 has an outerdiameter d6 of about 0.033 inches and a length L2 of about 0.031 incheswith a tapered or beveled region 60 with a taper angle θ of about 13degrees that extends beyond and overhangs at least a portion of theexpanded region 16. The region 40 between the bottom side of the ferrule30 and the right side of the expanded region 16 (and a portion of thefiber 10) may optionally be partially or completely filled with anadhesive, e.g., solder, braze, epoxy, etc., as discussed hereinbefore.The adhesive may also fill any gaps on the left side of the region 16.Other diameters, lengths and taper angles may be used. Also, the lengthof the tapered region 60 may be shorter or there need not be any taperedregion. The ferrule 30 may also be disposed within a housing 62, asdiscussed more hereinafter.

Referring to FIG. 6, alternatively, the ferrule 30 may be placed (orstraddled) across the expanded region 16. In that case, the ferrule 30may have a straight (cylindrical) inside diameter which is larger than,or equal to the diameter d3 of the expanded region 16 plus the thicknessof the coating 18 (if used). In that case, regions 52 between the bottomside of the ferrule 30 and one or both sides of the expanded region 16may be partially or completely filled with an adhesive, e.g., solder,braze, epoxy, etc., similar to those discussed hereinbefore.Alternatively, the ferrule 30 may be heated and/or pressure (or force)applied, e.g., by atmospheric (such as pressure and/or vacuum),mechanical (such as crimping), and/or mechanical techniques, or anyother techniques, across one or both sides of the expanded region 16 asindicated by arrows 54, which causes the ferrule 30 to conform to and beagainst at least a portion of the expanded region 16, as indicated bythe dashed line profile 56.

The ferrule 30 may be much longer than that shown in FIG. 6 to the right(as indicated by dashed lines 57) or to the left (as indicated by dashedlines 59) or along both sides, of the region 16, if desired. In thatcase, the ferrule 30 may be locally deformed to conform to one or bothsides of the region 16. Also, if one or both sides of the ferrule 30 aremade to conform to the region 16, one or more of the aforementionedadhesives may also be used. Also, such heating and/or applying ofpressure (or force) may be performed on the left and/or right sides ofthe region 16 together, e.g., with a single crimping tool, coining tool,or the like.

Referring to FIG. 7, in the event that the expanded region 16 has astraight geometry, such as that shown by the lines 17, the ferrule 30may be disposed adjacent to (or against) at least one of the verticaledges 17 of the expanded region 16. In that case, the ferrule 30 mayhave a region 70 that overlaps all or a portion of (or overhang beyond)the top of the expanded region 16, and/or a region 72, that extends onthe opposite side of the ferrule, which may have a tapered section 73,as discussed hereinbefore with FIGS. 3 and 4. Also, a corner 74 of theferrule 30 may be rounded to minimize damage to the outer surface of thefiber or coating 18 (if used), if the ferrule 30 is slid along the fiberto the expanded region 16. Alternatively, instead of having the verticaledge 17 on both sides of the expanded region 16, the side of theexpanded region 16 opposite from where the ferrule 30 contacts the edge17 (e.g., the right side) may be rounded or another geometry, asindicated by the dashed lines 13.

Referring to FIG. 8, in the event that the expanded region 16 has anotch 11, the ferrule 30 may have an inwardly protruding section (ortooth) 76, which fits within the notch 11 to lock the fiber 10 to theferrule 30. Also, the ferrule 30 may be a multi-piece ferrule (such asthat discussed hereinbefore). In that case, to hold the ferrule 30together, the ferrule 30 may be self-locking or there may be a collar 78around the ferrule 30. Also there may be a raised section 80 (at eitherend of the ferrule 30) to provide a stop for the collar 78 or for otherpurposes. The notch 11 need not be centered along the expanded region16, and the size of the tooth 76 need not match the dimensions (e.g.,length, depth) of the notch 11. Also, there may be more than one notch11 and tooth 76. Further the length L2 of the ferrule 30 may extendbeyond the length L of the expanded region 16, but is not required to.

Referring to FIG. 9, alternatively, if the region 16 comprises a recess8, the tooth 76 of the ferrule 30 would be sized to substantially matchat least a portion of the geometry of the recess 8. For example, if thegeometry of the recess 8 is curved, as indicated by the dashed lines 9,the tooth 76 of the ferrule 30 would likely also be curved. If thegeometry of the recess 8 has sharp edges 2, the tooth 76 may likely haveat least one sharp edge to match at least one of the edges 2. Also thelength of the tooth 76 may be shorter than the length of the recess 8.Further, the length L2 of the ferrule 30 may be longer than the length Lof the recess 8. In that case, there may be one or more tapered surfaces82, similar to that discussed hereinbefore, to reduce fiber stresses.

Referring to FIG. 10, alternatively, if the region 16 has the recess 8,the ferrule 30 maybe a single or multi-piece cylindrical tube (orsleeve), which is placed (or slid) over the recess 8. In that case, aregion 84 between the inside of the ferrule 30 and the outside of therecess 8 may be partially or completely filled with an adhesive, e.g.,solder, braze, epoxy, etc., similar to those discussed hereinbefore.Instead of using an adhesive, the ferrule 30 may be heated and/orpressure applied across the recess 8., e.g., by atmospheric (such aspressure and/or vacuum), mechanical (such as crimping), and/or magnetictechniques (such as electromagnetic forming), or any other technique, asindicated by arrows 90, which causes the ferrule 30 to conform to atleast a portion of the shape of the recess 8, as indicated by the dashedline profile 92. For a glass ferrule, the ferrule 30 may be heated to atemperature at or below the softening temperature of the glass. For ametal ferrule, the ferrule may be heated to a temperature where themetal can be shaped.

The ferrule 30 of any of the embodiments discussed herein may beconnected to or part of a structure (or housing), as discussedhereinbefore in the Background Art section hereto. Various techniquesfor attaching the ferrule 30 to the structure may be used, which dependon the application and the material of the ferrule 30.

For example, referring to FIG. 5, a housing 62 may surround at least aportion of the ferrule 30 to hold the ferrule 30 in a predeterminedposition. The housing 62 has a notch 64, which is substantially the samelength or longer than the length L2 of the ferrule 30. The depth d7 ofthe notch 64 is deep enough to hold the ferrule 30 from moving axially(in at least one direction). Also, the depth d7 may be deep enough toalmost touch the fiber 10 (which may reduce non-axial motion of thefiber 10). The shape of the housing 62 and the notch 64 may becylindrical, rectangular or any other shape that allows the notch 64 tohold the ferrule 30. The housing 62 may also be bonded to the ferrule 30using an adhesive discussed hereinbefore (e.g., solder, braze, epoxy,etc.). Also, the housing 62 may be anchored to the ferrule 30 bymechanical means, such as one or more set screws 66. Other techniquesfor attaching the ferrule 30 to the housing 62 may be used. The housing62 may be used with any of the ferrules 30 discussed herein withsuitable changes for the ferrule geometry.

One technique for making the expanded region 16 in the optical fiber 10is to use a fiber (or fiber section), which has an enlarged diameter d4substantially equal to or greater than the diameter d3 of the region 16.The fiber section may be made using a suitable glass pre-form with acladding/core diameter ratio that can be drawn down using conventionaltechniques to achieve the desired core size but has a cladding outerdiameter d4 which is greater than the desired value for the finaloptical fiber. To create the expanded region 16, as shown in FIG. 11,the diameter d4 of the fiber 10 is reduced to the desired diameter byeliminating an outer portion 15 of the cladding by conventional (or yetto be developed) glass manufacturing techniques, e.g., grinding,etching, polishing, etc. If desired, some of the outer diameter of theregion 16 may also be removed. Using chemical etching (e.g., withhydrofluoric acid or other chemical etches), laser etching, or laserenhanced chemical etching are some techniques, which reduce the fiber'souter diameter without applying direct contact force, as is required bygrinding and polishing. Certain types of etching may produce a sharpervertical edge 17 on the region 16, or an angled or curved edge 13. Also,selective etching may produce a notch 11 (or more than one notch) in theregion 16 (see FIG. 8). Also, the etching may produce the sharp edge 17at one side (e.g., the left side) of the region 16 and the curvedgeometry 13 on the other side (e.g., the right side) of the region 16,as shown in FIG. 7.

Fire polishing using conventional techniques, i.e., applying heat for apredetermined time across the region 16, may be performed after theetching to smooth any rough surfaces that may be left by the etchingprocess (as rough surfaces may increase stress levels and reduce fatiguelife in dynamically loaded fibers). The fiber section may then beoptically connected, e.g., by fusion splicing, by an optical connector,etc. to a standard-sized fiber (not shown) having a cladding and corewhich match the final fiber section described hereinbefore.

Referring to FIG. 12, alternatively, instead of the region 16 being madeusing a single axially continuous fiber, a fiber 4 having a length L andan outer diameter dy e.g., 125 microns, is fusion spliced between twofibers 3 having an outer diameter dx, e.g., 80 microns, at interfaces5,6. The fibers 3,4 have the same core 14 diameter, e.g., 9 microns, andmay be fusion spliced using known splicing techniques. Other diametersfor the claddings and cores of the fibers 3,4 may be used. The edge 17may be a vertical edge or may be a curved edge, as shown by the dashedlines 13. Depending on the application, it may be desirable and/oracceptable to have only one change in the outer dimension of the fiber(or two changes located a long distance apart). In that case, therewould be one splice, e.g., at the interface 5, between the fibers 3,4and the fiber 4 would be longer than that shown in FIG. 12.

Referring to FIG. 13, alternatively, a glass/ceramic tube (or sleeve) 7may surround the fiber 10 to create the expanded region 16. In thatcase, the tube 7 is heated to the melting or softening temperature ofthe tube 7 such that the tube 7 is fused to or becomes part of thecladding 12. The tube 7 has a softening temperature, which is the sameas or slightly lower than that of the fiber 10. Any form of heating maybe used, e.g., oven, torch, laser filament, etc. The tube 7 may be asingle cylindrical piece or have multiple pieces to surround the fiber10. To help keep the tube concentric with the fiber, the process may beperformed with the fiber held vertically. Also, more than one concentrictube may be used around the fiber if desired, each tube being meltedonto an inner tube at the same time or successively.

Referring to FIG. 14, alternatively, instead of the region 16 being anexpanded outer dimension (or diameter), the region 16 may comprise adecreased outer dimension (or recess or depression or notch) 8 in thewaveguide 10. The recess 8 may be created by numerous techniques, suchas by reducing the outer diameter of the fiber 10 using the techniquesdiscussed hereinbefore with FIG. 11 (e.g., grinding, etching, polishing,etc.), by splicing a smaller diameter fiber between two larger diameterfibers, such as that discussed hereinbefore with FIG. 12, or by heatingand stretching the desired region of the fiber by pulling on one or bothends of the fiber 10 (i.e., putting the fiber 10 in tension) using atechnique similar to that for heating and compressing the fiber tocreate a bulge in the fiber 10 (i.e., stretching instead ofcompressing), such as is described in conjunction with FIG. 15. Etchingthe fiber 10 may create recessed vertical edges 2 (into the fiber 10) ora curved or angled recessed geometry 9, and heating and stretching thefiber 10 creates the curved geometry 9. The depth d8 of the recess 8 maybe the same as the distance the expanded region 16 in FIGS. 3-5 extendsfrom the cladding 12 diameter, e.g., about 75 microns. Other depths maybe used.

If heating and stretching are used to create the recessed region 8, sucha process may be performed with the longitudinal axis of the fiber 10aligned horizontally or vertically or with other orientations. Oneadvantage to vertical orientation is that it minimizes axial distortionscaused by gravitational effects of heating a fiber. Alternatively, thefiber may be rotated during heating and stretching to minimize gravityeffects.

For any of the embodiments described herein, precise symmetry (axial orcross-sectional) of the region 16 (for either expanded or recessedregions) are not required for the present invention. For example, thelower portion of the regions 16,8 may be slightly larger or smaller thanthe upper portion, or vise versa. However, the core 14 should retainaxial alignment along both sides of the region 16 (or 8) to minimizeoptical losses from the core 14 as light travels through the region 16.The better the axial alignment of the core 14, the lower the opticalloss. Although the core 14 at the region 16 are shown as being straight,it should be understood that there may be some small amount ofdeformation of the core 14. The less deformation of the core 14 at theregion 16, the lower the amount of optical loss. We have measured totaloptical losses as low as 0.06 dB; however, lower losses may be achieved.The better the axial alignment of the core 14, the lower the opticalloss. Although the core 14 at the expanded region 16 is shown as beingstraight, it should be understood that there may be some small amount ofdeformation of the core 14. The less deformation of the core 14 at thebulge location, the lower the amount of optical loss. Also, the strengthof the fiber 10 remains strong after the expanded region 16 is created.For example, we have measured a proof force of up to 4.66 lbs. of axialtension force on the fiber 10 before breakage occurs, which iscomparable to a good fusion splice. Other fiber strengths may beobtained depending on the settings and method used to make the expandedregion 16.

Also, for any of the embodiments described herein, instead of an opticalfiber 10, any optical waveguide having a core and cladding may be used,e.g., a flat or planar waveguide, on which the region 16 can be created.In the case of a flat or planar waveguide, the region 16 may be on theupper and/or lower surfaces of the waveguide. Also, a multi-mode opticalwaveguide may be used if desired.

The region 16 may have other shapes (or geometries) than those describedherein, provided at least a portion of the optical waveguide has avariation, deformation or change (expanded and/or recessed) of the outerdimension of the waveguide.

Also, a combination of any of the above techniques for creating theregion 16 may be used. For example, the etching technique discussed inconjunction with FIG. 11 may be used to alter the geometries describedwith FIGS. 12-14. Other techniques than those described herein may beused if desired to create the region 16.

Also, the region 16 described with FIGS. 11-14 may be combined toprovide both an expanded outer diameter region and a reduced diameterregion. Further, more than one of the regions 16 may be provided along agiven optical fiber if desired.

After the regions 16 are made, the cladding 12 may be coated orre-coated with a protective overcoat or buffer layer (see FIG. 3, forexample), such as a metal, polymer, teflon, and/or carbon, or othermaterials, which may be used to protect the fiber and/or enhanceattachment to the fiber.

Referring to FIGS. 15 and 16, one technique for making the expandedregion 16 in the optical fiber 10 is to heat and compress the fiber 10as follows. First, the fiber 10 is prepared by stripping any protectiveover-coating or buffer layers from the fiber 10 to expose the cladding12 of the fiber 10 in at least the area where the expanded region 16 isto be made. This may be done by chemical or thermal techniques, such asdipping the desired section of the fiber in a hot bath of sulfuric acid.Then, the fiber is cleaned using well known procedures in the field ofoptical splicing, such as dipping in deionized water and then inisopropyl alcohol. Other stripping and/or cleaning techniques may beused if desired, providing they do not damage the fiber.

Referring to FIGS. 15 and 16, a device 100 that may be used to make theexpanded region 16 is a Model FFS-1000 Filament Fusion Splicing System,made by Vytran Corp. The device 100 comprises a pair of movable fiberholding blocks 23, a pair of vacuum V-groove fiber holders 22, a movablesplice head 25 and a hinge-mounted splice top 24 with a filament porthole 26. The fiber holding blocks 23 comprise a U-shaped frame and acenter, spring-loaded block that contains a vacuum V-groove insert, inwhich the fiber is inserted. The components 22,23 are aligned such thatthe fiber 10 lies substantially along a straight line. Within each ofthe fiber holding blocks 23, a stepper motor-driven worm-gear rotarymechanism (not shown) allows for movement of the blocks 23 (and thus thefiber 10) along the longitudinal axis of the fiber 10. The parts 22-26are supported by a transfer jig or housing 27. The splice head 25comprises a heat source, e.g., a resistive heating element (such as aTungsten filament ribbon) 29 (FIG. 16) having a width W of about 0.025inches, which provides radiation heating evenly around the circumferenceof the fiber 10. Other heating techniques may be used if desired, e.g.,a laser, a small oven, a torch, etc. Also, other devices and componentsfor aligning and axially compressing the fiber 10 may be used, if sodesired.

The fiber 10 is placed in the blocks 23 and the holders 22 (and acrossthe splice head 25), which places the longitudinal axis of the fiber 10substantially along a straight line, i.e., in axial alignment (along thelongitudinal or Z-axis of the fiber). The vacuum in the vacuum V-groovefiber holders 22 is set strong enough to keep the fiber in axialalignment but not so strong as to cause surface defects on the fiber.Next, the fiber 10 is heated where the bulge is to be made by applying apredetermined amount of power to the filament 29, e.g., about 26 Wattspower. The heating element reaches a temperature (approximately 2100°C.), such that the glass is at about 2000° C. (the melting or softeningtemperature of the glass fiber). The heat is applied to the fiber for aduration (pre-heat time) long enough to soften the fiber 10 enough to becompressed, e.g., approximately one second.

Then, while heat is still being applied to the fiber 10, the fiber 10 iscompressed axially by translation of the blocks 23 toward each other asindicated by the arrows 21 by the motors within the blocks 23. The totaltranslation of the blocks 23 (and thus compression of the fiber 10) isabout 400 microns at a rate of 100 microns/sec for about 4 seconds.Other compression amounts, rates, and times for the axial compressionmay be used, if so desired. Compression may be achieved by moving one orboth blocks 23 provided the same total motion occurs. After thecompression is complete, the heating of the fiber may be maintained fora predetermined post-compression time, approximately 0.25 seconds, toallow the expanded region 16 to reach final form. Other pre-heat timesand post-compression times may be used.

Next, the fiber 10 is again heated with the filament 29 (or “firepolished”) to remove surface defects, at a power setting of about 21.5Watts. During fire polishing, the filament (and the splice head 25) ismoved back and forth (e.g., 2 full passes) across a predetermined lengthof the fiber (about 2500 microns) across where the expanded region 16was formed, as indicated by the arrows 19, for a duration of about 3seconds. Other fire polishing power (temperature), number of passes, andtime settings may be used, if so desired, provided the surface defectsare removed and the expanded region 16 is substantially not altered ordeformed. The fire polishing may be performed immediately after formingthe expanded region without stopping the heating of the fiber, or theheating of the fiber may be stopped (filament turned off) for apredetermined period of time after compression is complete and thenturned on to perform the fire polish.

Also, during heating, the area within the splice head 25 around thefiber 10 is purged with flowing high purity argon gas to keep the fiberclean and to prevent high temperature oxidation of the tungstenfilament.

The parameter settings (times, powers, etc.) described above result inan acceptable combination of mechanical strength and low optical loss.However, other suitable parameter combinations may be used, if desired,to obtain a similar effect, which may be determined by one skilled inthe art in view of the teachings herein.

The process described for making the expanded area 16 may be performedwith the longitudinal axis of the fiber 10 (and the device 100) alignedhorizontally or vertically or with other orientations. One advantage tovertical orientation is that it minimizes axial distortions caused bygravitational effects of heating a fiber. Alternatively, the fiber maybe rotated during heating and compression to minimize gravity effects.

After the expanded area 16 is made, the cladding 12 may be re-coatedwith the protective overcoat or buffer layer 18 (see FIG. 3, forexample), such as a metal, polymer, teflon, and/or carbon, or othermaterials.

The ferrule 30 may have other shapes, sizes, and/or designs than thosedescribed herein, that has at least a portion of the ferrule 30 thatmechanically locks, stops, or otherwise is disposed against at least aportion of the variation region 16 (or 8), so as to minimize (orsubstantially prevent) relative movement (or creep) in at least onedirection between the fiber 10 and the ferrule 30 (i.e., substantiallyprevents the fiber 10 from moving in a predetermined direction relativeto the ferrule 30 and substantially prevents the ferrule 30 from movingin a direction opposite to the predetermined direction relative to thefiber 10), which causes the fiber 10 to substantially track movement ofthe ferrule 30. Also, the ferrule 30 may be placed against the rightside of the expanded region 16 instead of, or in addition to, the leftside of the expanded region 16.

Also, instead of a ferrule 30, the region 16 may be placed in a housingor any other structure having an internal shape that mechanically locks,stops, or otherwise is disposed against at least a portion of thevariation region 16, which minimizes relative movement (or creep) in atleast one direction between the fiber 10 and the ferrule 30. Also,although the fiber 10 and ferrule 30 are shown herein as being orientedhorizontally, the invention will work independent of the orientation ofthe fiber 10 and the ferrule 30, e.g., vertical, horizontal, or anyother orientation.

Referring to FIG. 17a in the embodiment, as described in conjunctionwith FIGS. 1 to 2 b, two or more Bragg gratings 184, 186 may be impartedin the fiber core 14 between the variation regions 16, 16′ for tuning.As such, at least one Fabry-Perot arrangement is achieved in the cavity185 between the Bragg gratings 184, 186. Accordingly, one or more fiberlasers, such as that described in U.S. Pat. No. 5,666,372,“Compression-Tuned Fiber Laser” (which is incorporated herein byreference to the extent necessary to understand the present invention)may be embedded within the fiber 10 between the variation regions 16,16′ for tuning. It is understood that a rare earth dopant, e.g., erbiumand/or ytterbium, is doped in at least one part of the fiber core 14,including the cavity 185.

Alternatively, as shown in FIG. 17b, a single Bragg grating 187 isimparted in the core 14 and at least one section of the core, includingthe Bragg grating, is doped with a rare earth dopant for achieving atunable distributed feedback (DFB) fiber laser 189, such as thatdescribed in V. C. Lauridsen et al., “Design of DFB Fiber Lasers”(Electronic Letters, Oct. 15, 1998, Vol.34, No. 21, pp 2028-2030); P.Varming, et al, “Erbium Doped Fiber DFB Laser With Permanent π/2Phase-Shift Induced by UV Post-Processing”, (IOOC'95, Tech. Digest, Vol.5, PD 1-3, 1995); U.S. Pat. No. 5,771,251, “Optical Fibre DistributedFeedback Laser”, to Kringlebotn et al; or U.S. Pat. No. 5,511,083,“Polarized Fiber Laser Source”, to D'Amato et al. In that case, thegrating 187 is written in a rare-earth doped fiber and configured tohave a phase shift of λ/2 (where λ is the lasing wavelength) at apredetermined location 188 near the center of the grating 187 whichprovides a well defined resonance condition that may be continuouslytuned in single longitudinal mode operation without mode hopping, as isknown. Alternatively, instead of a single grating, the two gratings184,186 (FIG. 17a) may be placed close enough to form the cavity 185having a length of (N+½)λ, where N is an integer (including 0) and thegratings 184,186 are in rare-earth doped fiber.

Alternatively, the DFB laser 189 may be located on the fiber 10 betweenthe pair of gratings 184,186 (FIG. 17a) where the fiber 10 is doped witha rare-earth dopant along at least a portion of the distance between thegratings 184,186. Such configuration is referred to as an “interactivefiber laser”, as is described by J. J. Pan et al., “Interactive FiberLasers with Low Noise and Controlled Output Power”, E-tek Dynamics,Inc., San Jose, Calif., internet websitewww.e-tek.com/products/whitepapers and U.S. Pat. No. 6,018,534, entitled“Fiber Gragg Grating DFB-DBR Interactive Laser Sources” to Par et al.Other single or multiple fiber laser configurations may be disposed onthe fiber 10 if desired.

It should be noted that the frame 120, as shown in FIGS. 1, 2 a, 2 b, 17a-17 b, is an enclosed frame. However, the frame 120 can also be an openframe. For example, the top section 123 of the frame 120 (see FIG. 1)can be removed.

It should also be noted that the various embodiments of the presentinvention can be combined with various temperature compensation designs,such as those described in U.S. patent application, Ser. No. 09/519,240,filed Mar. 6, 1999 by Richard T. Jones et al., which is incorporatedherein by reference in its entirety.

The present invention has numerous applications. For example, it can beused in an optical instrument, wherein a tunable filter is needed. Itcan also be used in an optical scanner.

It should be understood that any of the features, characteristics,alternatives or modifications described regarding a particularembodiment herein may also be applied, used, or incorporated with anyother embodiment described herein.

Although the invention has been described and illustrated with respectto exemplary embodiments thereof, the foregoing and various otheradditions and omissions may be made therein and thereto withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A tunable optical device comprising: an opticalwaveguide having a longitudinal axis, a first mounting location and asecond mounting location separated by a distance along the longitudinalaxis, which transmits an optical signal, wherein the waveguide comprisesa core and a cladding disposed outside the core, and wherein thecladding has an outside diameter and includes a first and a secondvariation region each having a modified outside diameter different fromthe outside diameter, wherein the first and second variation regions arerespectively located at the first mounting location and the secondmounting location; a Bragg grating imparted in the core of the waveguidebetween the first mounting location and the second mounting location,wherein the Bragg grating comprises a plurality of perturbations definedby a spacing along the longitudinal axis to partially reflect thetransmitted optical signal at a reflection wavelength characteristic ofthe spacing of the Bragg grating; a first attachment mechanism disposedagainst at least one portion of the first variation region whichprevents relative movement between the first variation region and thefirst attachment mechanism; a second attachment mechanism disposedagainst at least one portion of the second variation region whichprevents relative movement between the second variation region and thesecond attached mechanism; a mounting device having a first end forfixedly mounting the first attachment mechanism and a second end whichmovably mounts to the second attachment mechanism and defines aseparation length between the first and second attachment mechanismsalong the longitudinal axis of the waveguide; and an adjustmentmechanism, operatively connected to the second attachment mechanism,which adjusts the separation length, thereby causing a change in thedistance between the first and second variation regions and the spacingof the Bragg grating to tune the reflection wavelength.
 2. The tunableoptical device of claim 1, wherein the first attachment mechanismcomprises: a first ferrule including a front portion having a profilesubstantially corresponding to the modified outside diameter of thefirst variation region of the cladding; and a first butting mechanismbutting the first ferrule against the waveguide to press the frontportion of the first ferrule onto at least one portion of the firstvariation region at the first mounting location which limits relativemovement between the first ferrule and the first variation region of thecladding; and the second attachment mechanism comprises: a secondferrule including a front portion having a profile substantiallycorresponding to the modified outside diameter of the second variationregion of the cladding; and a second butting mechanism butting thesecond ferrule against the waveguide to press the front portion of thesecond ferrule onto at least one portion of the second variation regionat the second mounting location which limits relative movement betweenthe second ferrule and the second variation region of the cladding. 3.The tunable optical device of claim 2, wherein the first buttingmechanism provides a pressing force against the front portion of thefirst ferrule along a first direction substantially parallel to thelongitudinal axis, and the second butting mechanism provides a pressingforce against the front portion of the second ferrule along a seconddirection substantially opposite to the first direction.
 4. The tunableoptical device of claim 1, wherein the adjustment mechanism comprises apiezoelectric actuator.
 5. The tunable optical device of claim 1,wherein the adjustment mechanism comprises a motor.
 6. The tunableoptical device of claim 1, wherein the first and second attachmentmechanisms have two collars, each which holds one of the attachmentmechanisms against the waveguide.
 7. The tunable optical device of claim1, wherein the first and second variation regions include expandedregions in the cladding such that the modified outside diameter isgreater than the outside diameter of the cladding.
 8. The tunableoptical device of claim 1, wherein the first and second variationregions include recessed regions in the cladding such that the modifiedoutside diameter is smaller than the outside diameter of the cladding.9. The tunable optical device of claim 2, further having a coatinglocated between the cladding and the first and second ferrules whichhelps the ferrules to conform with the outside diameter of therespective variation regions which reduces point contact stresses on thewaveguide.
 10. The tunable optical device of claim 1, wherein the firstand second attachment mechanisms overhang at least one portion of therespective variation regions.
 11. The tunable optical device of claim 1,wherein the waveguide further comprises a buffer layer over the claddingto protect the waveguide against the first and second attachmentmechanisms and which enhances attachment of the first and secondattachment mechanisms to the waveguide.
 12. The tunable optical deviceof claim 11, wherein the first and second attachment mechanisms arebonded to the buffer layer.
 13. The tunable optical device of claim 11,wherein the buffer layer comprises a metal layer.
 14. The tunableoptical device of claim 13, wherein the metal layer comprises Ni and Au.15. The tunable optical device of claim 11, wherein the buffer layercomprises a polymer layer.
 16. The tunable optical device of claim 2,wherein the first and second ferrules comprise a plurality of piecessubstantially surrounding the respective variation regions, which attachto the cladding.
 17. The tunable optical device of claim 1, wherein themodified outside diameter causes minimal deformation to the core inorder to minimize optical loss from the core due to the modified outsidediameter.
 18. The tunable optical device of claim 1, wherein the core ofthe waveguide is axially continuous near the first and second variationregions.
 19. The tunable optical device of claim 1, further comprising afurther waveguide segment including a cladding having a second outsidediameter substantially equal to the modified outside diameter to splicewith the waveguide in order to provide each of the first and secondvariation regions.
 20. The tunable optical device of claim 1, whereinthe modified outside diameter is provided by fusing a tube to thecladding.
 21. The tunable optical device of claim 1, wherein themodified outside diameter is provided by heating and stretching thewaveguide to change the outside diameter of the cladding.
 22. Thetunable optical device of claim 1, wherein the modified outside diameteris provided by etching the outside diameter of the cladding.
 23. Thetunable optical device of claim 1, wherein at least one portion of thecore between the first and second mounting locations is doped with arare-earth dopant to form a distributed feedback laser with the Bragggrating.
 24. The tunable optical device of claim 1, further comprising afurther Bragg grating adjacent to the Bragg grating to form a cavitytherebetween, wherein at least one portion of the core between the firstand second mounting locations is doped with a rare-earth dopant to forma laser using the cavity.
 25. A method of wavelength tuning an opticaldevice, wherein the optical device comprises: an optical waveguidehaving a longitudinal axis to transmit an optical signal, wherein thewaveguide has a first mounting location and a second mounting locationseparated by a distance along the longitudinal axis, and wherein thewaveguide comprises a core and a cladding disposed outside the core,wherein the cladding has an outside diameter and includes a first and asecond variation region each having a modified outside diameterdifferent from the outside diameter, and wherein the first and secondvariation regions are respectively located at the first mountinglocation and the second mounting location; and a Bragg grating impartedin the core of the waveguide between the first mounting location and thesecond mounting location, wherein the Bragg grating comprises aplurality of perturbations defined by a spacing along the longitudinalaxis to partially reflect the transmitted optical signal at a reflectionwavelength characteristic of the spacing of the grating, said methodcomprising the steps of: providing a first attachment mechanism disposedagainst at least one portion of the first variation region whichprevents relative movement between the first variation region and thefirst attachment mechanism; providing a second attachment mechanismdisposed against at least one portion of the second variation regionwhich prevents relative movement between the second variation region andthe second attached mechanism; providing a mounting device having afirst end which fixedly mounts to the first attachment mechanism and asecond end which movably mounts to the second attachment mechanism inorder to define a separation length between the first and secondattachment mechanisms along the longitudinal axis of the waveguide; andproviding an adjustment mechanism, operatively connected to the secondmechanism, to adjust the separation length, thereby causing a change inthe distance between the first and second variation regions and thespacing of the grating which tunes the reflection wavelength.
 26. Themethod of of claim 25, wherein the first attachment mechanism comprises:a first ferrule including a front portion having a profile substantiallycorresponding to the modified outside diameter of the first variationregion of the cladding; and a first butting mechanism butting the firstferrule against the waveguide to press the front portion of the firstferrule onto at least one portion of the first variation region at thefirst mounting location in order to limit relative movement between thefirst ferrule and the first variation region of the cladding, and thesecond attachment mechanism comprises: a second ferrule including afront portion having a profile substantially corresponding to themodified outside diameter of the second variation region of thecladding; and a second butting mechanism butting the second ferruleagainst the waveguide to press the front portion of the second ferruleonto at least one portion of the second variation region at the secondmounting location in order to limit relative movement between the secondferrule and the second variation region of the cladding.
 27. The methodof claim 26, wherein the first butting mechanism provides a pressingforce against the front portion of the first ferrule along a firstdirection substantially parallel to the longitudinal axis, and thesecond butting mechanism provides a pressing force against the frontportion of the second ferrule along a second direction substantiallyopposite to the first direction.
 28. The method of claim 25, wherein theadjustment mechanism comprises a piezoelectric actuator.
 29. The methodof claim 25, wherein the adjustment mechanism comprises a motor.
 30. Themethod of claim 25, further comprising the step of providing collarswhich hold the first and second attachment mechanisms against thewaveguide.
 31. The method of claim 25, wherein the first and secondvariation regions include expanded regions in the cladding such that themodified outside diameter is greater than the outside diameter of thecladding.
 32. The method of claim 25, wherein the first and secondvariation regions include recessed regions in the cladding such that themodified outside diameter is smaller than the outside diameter of thecladding.
 33. The method of claim 26, further comprising the step ofproviding a coating between the cladding and the first and secondferrules which helps the ferrules to conform with the outside diameterof the respective variation regions in order to reduce point contactstresses on the waveguide.
 34. The method of claim 25, furthercomprising the step of providing a buffer layer over the cladding whichprotects the waveguide against the first and second attachmentmechanisms and enhances attachment of the first and second attachmentmechanisms to the waveguide.
 35. The method of claim 34, furthercomprising the step of bonding the buffer layer to the first and secondattachment mechanisms.
 36. The method of claim 25, wherein the first andsecond ferrules comprise a plurality of pieces substantially surroundingthe respective variation regions which attach to the cladding at eachmounting location.
 37. The method of claim 25, further comprising thestep of splicing a further waveguide segment including a cladding havinga second outside diameter substantially equal to the modified outsidediameter with the waveguide to form each of the first and secondvariation regions.
 38. The method of claim 25, further comprising thestep of fusing a tube to the cladding to form the modified outsidediameter of the first and second variation regions.
 39. The method ofclaim 25, further comprising the step of heating and stretching thewaveguide to form the modified outside diameter of the first and secondvariation regions.
 40. The method of claim 25, further comprising thestep of etching the outside diameter of the cladding to form themodified outside diameter of the first and second variation regions.